1. Field of the Invention
The present invention relates to a bead coating method and apparatus for applying a coating composition to a continuously traveling web. It more particularly relates to a bead coating method and apparatus used for manufacturing photographic light-sensitive materials, recording materials, and the like.
2. Background of the Invention
Heretofore, bead coating methods and apparatus have been generally used for manufacturing photographic light-sensitive materials. Typical examples of the methods and apparatus are a slide hopper type bead coating method and apparatus, and an extrusion hopper type bead coating method and apparatus.
In those bead coating methods, a large quantity of coating composition must be supplied for forming beads at the start of coating. Hence, a thick coating portion is often generated. Further, the coating head must be moved away from the web when it is passed by a spliced portion of the web. Hence, bubbles are attached to the spliced portion so that a longitudinal strip occurs. Further, when the coating composition is applied, the thick coating portion is often generated. Various types of measures to eliminate the above defects have been proposed.
Examples of the measure to eliminate the generation of the thick coating portion include the following methods. One method improves the wetness of the coating composition, such as by roughing the web surface to be coated, by moistening the web surface, by applying corona discharge to the web surface, and the like.
Other methods improve the reduced pressure within a suction chamber for stabilizing coating beads. Reference is made to U.S. Pat. No. 3,220,877, Japanese Patent Unexamined Publication No. 92328/1975, Japanese Patent Unexamined Publication no. 31727/1977, Japanese Patent Publication No. 36025/1982, Japanese Patent Unexamined Publication No. 119470/1980, Japanese Patent Unexamined Publication No. 121865/1980, and Japanese Patent Unexamined Publication No. 142565/1980.
Examples of the measures to prevent the longitudinal stripe due to the attachment of bubbles to the spliced portion include the followings. One set of methods use improved splicing tapes for preventing the attachment of bubbles. Another set of methods apply hydrophobic liquid to the junction portion. Reference is made to Japanese Patent Publication No. 42725/1972, Japanese Patent Publication No. 4371/1973, Japanese Patent Unexamined Publication No. 40638/1975, Japanese Patent Unexamined Publication No. 43140/1975, Japanese Patent Unexamined Publication No. 34343/1979 and Japanese Patent Unexamined Publication No. 62241/1979. Recently, a method has been disclosed in which the coating head supporting the hopper is moved away from the backing roller when the spliced portion is passed through the coating bead portion. Reference is made to Japanese Patent Unexamined Publication No. 88074/1983.
In any one of the conventional coating methods, the coating head supporting the hopper must be moved relative to the backing roller supporting the travelling web in order to approach the web or to retreat from the web.
The coating head is constituted by a hopper and a pedestal for supporting the hopper. The hopper must be formed of a specific alloy having a small coefficient of thermal expansion in order to maintain accuracy so that the hopper becomes considerably large in volume and heavy in weight. Further, the pedestal supporting the hopper must be protected against vibration from the outside so that the pedestal becomes considerable heavy in weight. Accordingly, in the case where such heavy weight elements are rapidly moved at the start of coating or at the time of being passed by the spliced portion, a limit exists for both the distance and speed because of inertia. Accordingly, there is a limit in the minimum coating quantity for a coating process. Further, the time required for supplying the coating composition at a process of coating becomes long. Consequently, there is a limit in preventing a thick coating at the start of coating and preventing bubble attachment and thick coating at the time of being passed by the spliced portion. Thus, the loss of length in the products cannot be disregarded.